Turning to Ukraine's European aspirations, Poroshenko said he intends very soon to sign the economic part of an EU Association Agreement and sees it as a first step toward eventual full membership in the EU.

Among those attending the June 7 ceremony were U.S. Vice President Joe Biden, Belarus President Alyaksandr Lukashenka, and the Russian charge d'affaires in Kyiv.

Poroshenko succeeds Viktor Yanukovych, who was impeached by parliament in February after fleeing Kiev following three months of mass protests against his government.

Hours ahead of the inauguration ceremony in Kyiv, Ukrainian government forces were continuing to battle pro-Russian separatists in the east near Slovyansk and Donetsk.

The inauguration comes a day after Poroshenko met briefly and unofficially in France with Russian President Vladimir Putin during a commemoration of the D-Day invasion's 70th anniversary.